The Physicochemical Basis And Genetic Characteristics Of Favorable Eating Quality In Indica Hybrid Rice

China is the first country to use heterosis successfully for commercial rice,and the development of hybrid rice has made great contribution to the food security of our country and the world.With the continuous progress of society,"eating well"has become a common demand after"eating full".On the premise of meeting the basic requirements of high yield and good adaptability,"good taste"determines the market competitiveness of hybrid rice varieties.The genetic basis of eating quality of hybrid rice is more complex than that of conventional rice.The endosperm traits of hybrid rice are not only affected by both parents,and the edible part of hybrid rice is the F₂segregation population.At present,great progress has been made in the studies on the eating quality of conventional rice.However,there is little research on hybrid rice in this area,which can not provide theoretical support for quality breeding.This study systematically analyzed the physicochemical foundation for eating quality of indica hybrid rice and the influence of rice starch synthesis key genes on the eating quality of hybrid rice,which provide scientific reference for hybrid rice quality breeding.The main results are as follows:1.Correlations between parental lines and indica hybrid rice in terms of eating quality traitsBy analysing the physicochemical properties and molecular structures of starch in several hybrid combinations with different taste values.We found that the taste value was positively correlated with rapid visco-analyser（RVA）peak viscosity,breakdown values,gel consistency,stickiness of cooked rice and short-branch amylopectin chains but negatively correlated with apparent amylose content（AAC）,RVA setback values,protein content,hardness of cooked rice and amylose fractions.Furtherly,by analysing the relationship between hybrid combinations and parental lines,we found that the eating quality traits of the hybrid combinations were determined by both parents.The sterile lines determined the overall eating quality characteristics of the hybrid combinations.For the same sterile line,there were some correlations between the hybrid combinations and restorer lines in terms of taste value,RVA breakdown value and setback value,apparent amylose content,cooked rice hardness and stickiness,short-branch amylopectin chains and amylose fractions.2.Genetic effects of different allelic combinations of Wx and ALK genes on the eating quality of hybrid riceWx and ALK are the two key genes controlling the cooking and eating quality of rice.In this study,by analyzing the genetic effects of different allelic combinations of Wx on the eating quality of hybrid rice,we found that hybrid combinations with Wx^b/Wx^bshowed the highest taste value,stickiness,gel consistency,RVA breakdown value and peak viscosity,while the hardness,AAC,setback value,and final viscosity were lower than those of the other allelic combinations.However,the Wx^lv/Wx^bhybrid combinations showed the lowest taste value,stickiness,breakdown value,and peak viscosity as well as the highest hardness,AAC,and setback value.The genetic effects of Wx^a/Wx^awere similar to those of Wx^lv/Wx^bin terms of taste value and AAC,but with differences in some RVA viscosity characters,gel consistency and texture properties.The overall level of eating quality traits of Wx^b/Wx^aor Wx^a/Wx^bhybrid combinations were intermediate.We further analysed the genetic effects of different allelic combinations of ALK in the genetic background of Wx^b/Wx^b.It was found that the differences among allelic combinations of ALK were mainly reflected in the RVA pasting properties,while there were no significant differences in taste values and other physicochemical property.Overall,The eating quality of hybrid rice is mainly controlled by Wx gene,and the RVA viscosity characters were mainly regulated by different allelic combinations of Wx and ALK.3.Influence of physicochemical properties and starch fine structure on the eating quality of hybrid rice with similar apparent amylose contentThe AAC has been taken as a key factor affecting the eating quality of rice grains.However,during the quality breeding of hybrid rice,we found that hybrid rice varieties with similar AAC sometimes displayed differences in their eating quality.Concerning with this phenomenon,this study analyzed the physicochemical properties and starch molecular structure related to the eating quality of the hybrid combinations with similar AAC.Gel permeation chromatography（GPC）indicated that the hybrid rice with good taste values had higher proportions of amylopectin short chains but relatively lower proportions of amylopectin long chains,which could lead to higher peak viscosity and breakdown value,and a softer and stickier texture of the cooked rice.High-performance anion-exchange chromatography（HPAEC）analysis indicated that the proportion of degree of polymerization（DP）6-10 for the hybrid rice with good taste values was low,whereas the proportion of intermediate chains（DP 13-24）was high,which could lead to higher gelatinisation enthalpy and crystallinity.Moreover,for the first time,the present study indicated that the greater diversity of forms and sizes of starch granules might influence the eating quality of hybrid rice,the greater the difference of shape and size between starch granules,the lower taste value of hybrid rice.4.Allelic variations of the Wx locus in cultivated rice worldwide and their use in the development of hybrid rice in ChinaTo make better use of global germplasm resources for improving the eating quality of hybrid rice,using the resequencing data from the 3,000 rice genomes project（3K RGP）,the allelic variations of the rice Wx locus were analysed.Based on 1 insertion and 29 nucleotide polymorphic sites（SNPs）,a total of 30 haplotypes of Wx alleles were identified,including eight known Wx alleles and five rare alleles discovered for the first time.Furthermore,thirty-six main parents of hybrid rice from 1976 to 2018 were selected for Wx genotyping.The results showed that only three Wx alleles existed in the main parents of hybrids,and the allelic combination of the hybrids changed from Wx^a/Wx^band Wx^lv/Wx^bto Wx^b/Wx^bwith the development of hybrid rice.Wx^bis widely used in the male parents of hybrid rice.Wx^ais mainly existed in the male sterility of three-line hybrid rice,Wx^lvis mainly existed in the male sterility of early two-line hybrid rice,and gradually be replaced by Wx^b.In the future,more favourable Wx alleles from cultivated rice should be introduced for quality breeding of hybrid rice.